Method for the production of ductile and stable particle-superconductors

ABSTRACT

A method of the producing a ductile and stable particle superconductor, as well as the product itself, from particles of the formula NbN, NbN 1-x  C x  where x is less than or equal to 0.5, Nb 3  Al, Nb 3  Si, Nb 3  Ge, Nb 3  Sn, V 3  Ga or V 3  Si from a reaction in a hydrogen plasma. The particles are incorporated into a metallic matix or coprecipitated with a metal powder.

The invention refers to a method for the production of a ductile andstable particle superconductor.

The first particle superconductor was realized by Watson in App. Phys.Letters, 15 (125) 1969 by filling the pores of leached surfaces of Vycorglass by means of pressure with a lead melt. The pores had diameters of20 - 30 A and a distance of 10 - 20 A. The resulting composite showed acritical temperature slightly above that of bulk lead and values of theupper critical magnetic field H_(c2) tow orders higher. This indicatesthat lead, a superconductor of the first kind, was transformed into asuperconductor of the second kind simply by arranging superconductingparticles of 20 - 30 A diameter with a distance of 10 - 20 A in a nonsuperconducting matrix. The effect was explained by the ability of theelectrons in the form of Copper-pairs to tunnel the non-superconductingwalls between the superconducting particles. The theory of McMillan inPhys. Rev. 167, 331 (1968) as applied by the Watson Article and Hake inApp. Phys. Letters, 10, 189 (1967) to this special case gives thetheoretical background.

There have been many attempts to apply the principle of Watson toparticle superconductors with a metallic matrix. Particlesuperconductors consisting of small particles of brittle superconductingcompounds dispersed in a metallic matrix promise a combination of goodductility, high tensile strength and superior superconductingpreperties. Up to now the enhanced superconducting properties of Watsonsparticle superconductors with a glass matrix could not be realized in aparticle superconductor with a metal matrix. Tsuei in Science 180, 57(April 1973) tried to precipitate the superconducting particles from themelt and Mordike In J. Less-Common Metals, 11, 365-375 (1966), workedwith powdermetallurgical methods. The values of the critical temperatureT_(c) and the critical current density J_(c) of the bulk superconductorscould not be surpassed with the resulting composites. Neverthelesstheory as set forth in the Watson and McMillan articles, predictscritical temperatures T_(c) up to 40° K for V₃ Si-particles sufficientlysmall and closely arranged.

The reason for the unsuccessful attempts to realize a superior particlesuperconductor with a metallic matrix lies in the fact, thatsuperconducting particles with the required size of 20 - 500 A could notbe produced from the compounds of interest; furthermore the carefuldispersion of those particles in a non superconducting matrix wouldcause some problems.

Therefore in the following invention a method is given, which makes itpossible to produce these superconducting compounds on a large scale andin an economical way in the required size and form. Furthermore ways areshown to distribute these particles evenly with the required distance ina non superconducting marix; the resulting composites finally show thesuperconducting properties which can be expected from the theory.

The superconducting particles are compounds of niobium or vanadium asNbN, NbN_(x) C_(1-x) with 0.5 ≦ x ≦ 1 or A₃ B compounds with βW/A15structure; here A stands for Nb or V and B for Al,Ge,Si,Ga or Sn.

The compounds are produced by a reaction in the gas phase.Halogen-compounds of Nb or V are introduced in a reaction zone togetherwith hydrogen and either nitrogen or or halogen compounds of Al,Ge,Si,Gaor Sn.

A preferred method for the production of superconducting compounds withthe required size according to the invention is to introduce the halogencompounds of Nb or V into ahydorgen plasma together with N and/or ahydrocarbon like methane; this method was first described byNeuenschwander in J. Less-Common Metals, 11, 365-375(1966) for thepreparation of TiN or T a C as well as Nb with a particle size of about100 - 500 A. The hydrogen plasma with a temperature of about 3000° C isgenerated by an electric arc burning between a water cooled Wolframcathode and a similarly cooled copper anode; the copper anode has acentral boring through which the hydrogen is introduced. The electricalvalues for the arc are for instance 200 A and 120 V. The particlesresulting from the reaction fall out and are gathered on the floor ofthe enclosing container. Surprisingly it turned out that this methodexcellently can be employed for the mass production of A₃ B compoundswith βW/A15 structure, a fact of which Neuenschwander has not beenaware. Some of the reaction mehcanisms are presented in the followinglines:

    NbCl.sub.5 + 0.5N.sub.2 + 2.5H.sub.2 → NbN + 5HCl

    2NbCl.sub.5 + 0.5N.sub.2 + CH.sub.4 + 3H.sub.2 → 2Nb(N.sub.0.5 C.sub.0.5) + 10 HCl

    3NbCl.sub.5 + SnCl.sub.4 + 9.5 H.sub.2 → Nb.sub.3 Sn + 19 HCl

    3VCl.sub.5 + SiCl.sub.4 + 9.5 H.sub.2 → V.sub.3 Si + 19 HCl

Furthermore it became possible to bring down the size of the resultingparticles to 70 A by increasing the current and the voltage of the arcto 500 A and 150V. The particles were carefully stored under argon orhelium with a reduced pressure.

According to the invention, the incorporation of these finesuperconducting particles with a medium diameter of up to about 500 Ainto a ductile metallic matrix was accomplished by three differentmethods as described hereafter.

The first method is based on conventional powder-metallurgy. A carefullyprepared mixture of A₃ B/BW compounds as Nb₃ Sn or V₃ Si with a mediumparticle diameter below 500 A and a metal like copper with about thesame particle size was compacted, sheathed in a container of the samemetal, evacuated and closed; subsequently the compacts were extruded andfurther drawn into wires with a diameter of about 1 mm. using Nb₃ Snparticles with a medium diameter of 300 A and copper particles withabout 400 A and a volume of 35% Nb₃ Sn and 65% Cu, the resultingsuperconducting and mechanical values of the 1mm diameter wire after anannealing treatment at 700° C for 1 hour were:

    ______________________________________                                        T.sub.c                                                                            :  21° K                                                                            Tensile strength                                                                             : 74 kp/mm.sup.2                             H.sub.c2                                                                           : 470 kG     elongation     :  9%                                        J.sub.c                                                                            : 7 × 10.sup.5 A/cm.sup.2                                          ______________________________________                                    

The copper particles were produced by vacuum evaporation and storedunder vacuum; in contact with air they proved to be pyrophorous.

By comparison, using conventionally prepared Nb₃ Sn powders made by ballmilling the smallest size reached was about 20 000 A or 2 μm; mixingwith conventionally prepared copper powders with a diameter of about 3μm compacting the mixture and sheathing it in copper, after evacuatingand extruding and drawing into wires with about 1mm in diameter, thefollowing superconducting values after an annealing treatment of 1 hourat 700° C for a mixture of 35volume% Nb₃ Sn and 65 volume% Cu weremeasured:

    ______________________________________                                        T.sub.c                                                                            : 17.8° K                                                                           Tensile strength                                                                             : 57 kp/mm.sup.2                             H.sub.c2                                                                           : 180 kG     elongation     :  3 %                                       J.sub.c                                                                            : 2 × 10.sup.4 A/cm.sup.2                                          ______________________________________                                    

The second method chosen according to the invention for theincorporation of the minute superconducting particles into a metallicmatrix avoided the fire hazards and the tendency of contamination of thefine metal powders with the atmosphere; furthermore only 20 volume% ofsuperconducting particles in the metal matrix were necessary for optimumresults. About 300g of Nb₃ Sn particles with a mediumm diameter of 200 Awere suspended in an acid CuSO₄ electrolyte of about 1 liter(1000 cm³)with the help of a wetting agent like monolaurylsulfuric-acidester; bygalvanic deposition of the suspension electrolyte on a moving belt or arotating drum, both with a stainless-steel surface, the resultingparticles with a size of about 200 μm were continously scraped off anddried; they proved to be dispersion powders, that means, a finedispersion of regularly spaced Nb₃ Sn particles in a copper matrix;these powders were compacted, sheathed in a copper can, extruded andworked into semifinished material like plate, tube or wire. After anannealing treatment at 650° C for 1.5 hours, a tube with an outerdiameter of 20 mm and a wall thickness of 1 mm, suitable for conductingthe cooling medium like liquid helium, showed the followingsuperconducting and mechanical values employing a mixture of 20 volume%of Nb₃ Sn and 80 volume% of Cu:

    ______________________________________                                        T.sub.c                                                                            :  23° K                                                                            Tensile strength                                                                             : 78 kp/mm.sup.2                             H.sub.c2                                                                           : 570 kG     elongation     :  7%                                        J.sub.c                                                                            : 9 × 10.sup.5 A/cm.sup.2                                          ______________________________________                                    

The resulting tubes can be cooled with liquid hydrogen, leavingsufficient superconducting properties for technical use. The thirdmethod showed the finest distribution of superconducting particles in ametal matrix and employed so called duplex powders; the superconductingparticles falling out of the hydrogen plasma reaction zone were coveredat the same time with a layer of the metal by introducing a halogenideof the metal, like copper, into the reaction zone.

The resulting particles bearing the required layer thickness -- for the100 A particles of Nb₃ Sn or V₃ Si about 40 A -- were strictly keptunder argon or helium to avoid contamination with the atmosphere,compacted, canned into thin copper tubes, evacuated and extruded intosemifinished material like rods, tubes and wire. Using Nb₃ Sn particlesof about 100 A diameter and a copper layer of about 40 A, the resultingwire showed after an annealing treatment for 2 hours and 700° C thefollowing superconducting and mechanical values:

    ______________________________________                                        T.sub.c                                                                            : 24.7° K                                                                           Tensile strength                                                                             : 82 kp/mm.sup.2                             H.sub.c2                                                                           : 710 kG     elongation     :  7 %                                       J.sub.c                                                                            : 9 × 10.sup.5 A/cm.sup.2                                          ______________________________________                                    

Using 100 A particles of V₃ Si covered according to the invention with alayer of about 40 A of copper, the resulting wire showed superconductingfluctuations already at 29 K; here a broad transition range started. Themaximum upper critical magnetic field H_(c2) at 4.2° K nearly reached 1Megagauss.

Furthermore it lies in the scope of the invention, to cover metallic orelectrically conducting surfaces with superconducting layers by thegalvanic method of codeposition of a metal and superconducting particlesfrom a suspension electrolyte. Here too with diameters for thesuperconducting particles below 500 A superior superconductingproperties were realized compared with layers, incorporating coarsersuperconducting particles with diameters greater than 1000 A. Theresults were comparable with the given examples for the first and secondmethods.

What is claimed is:
 1. A method for the production of ductile and stableparticle-super conductors comprising the steps of:a. producingsuperconducting particles having an average particle diameter of 20 to500 A by reacting, in a hydrogen plasma,i. a halogenide of niobium withnitrogen to give NbN; ii. a halogenide of niobium with nitrogen andmethane to give NbN_(1-x) C_(x) with x being less than or equal to 0.5;iii. a halogenide of niobium with a halogenide of aluminum to give Nb₃Al; iv. a halogenide of niobium with a halogenide of silicon to give Nb₃Si; v. a halogenide of niobium with halogenide of germanium to give Nb₃Ge; vi. a halogenide of niobium with a halogenide of tin to give Nb₃ Sn;vii. a halogenide of vanadium and a halogenide of gallium to give V₃ Ga;or viii. a halogenide of vanadium with a halogenide of silicon to giveV₃ Si; b. incorporating the product particles produced in step (a) in ametallic matrix with distances between said particles between 20 and 500A; c. compacting the resulting metallic matrix; and d. fabricating theproduct produced from step (c) into semi-finished products.
 2. Themethod of claim 1, wherein said step (b) is accomplished by mixingpowders of said superconducting particles with powders of a metalselected from the group consisting of copper, silver, gold or aluminum.3. The method of claim 1, wherein said step (b) is accomplished bysuspending the superconducting particles in an electrolyte andcoprecipitating a metal powder selected from the group consisting ofcopper, silver, gold or aluminum and the superconducting particles inthe form of a powder dispersion by galvanic action.
 4. The method ofclaim 1, wherein said step (b) is acomplished by covering thesuperconducting particles with a layer of a metal selected from thegroup consisting of copper, silver, gold or aluminum during said step(a) by adding a metal vapor into the reaction zone.
 5. The method ofclaim 1, wherein said step (d) is accomplished by rolling.
 6. The methodof claim 1, wherein said step (d) is accomplished by extrusion.
 7. Themethod of claim 1, wherein said step (d) is accomplished by drawing. 8.The product produced by the method of claim
 1. 9. The method of claim 2wherein said particles produced in step (a) have a diameter of from 70to 500 A and said metal powder is copper which is used in an amount offrom 50 to 90 volume percent of the mixture.
 10. The method of claim 9,wherein said copper is manufactured by vacuum evaporation.
 11. Themethod of claim 3 wherein said susperconducting particles have adiameter of 70 to 500 A and are suspended in a copper electolyte in anamount up to 350 grams per liter and said coprecipitating is onto asurface of stainless steel.
 12. The method of claim 3, wherein a wettingagent is present in said electrolyte.
 13. The method of claim 4, whereinsaid metal is copper and is deposited onto said superconductingparticles by introducing CuCl₂ vapor into said reaction zone.
 14. Amethod for the production of ductile and stable particle-superconductors comprising the steps of:a. producing superconductingparticles having an average particle diameter of 20 to 500 A byreacting, in a hydrogen plasma,i. a halogenide of niobium with nitrogento give NbN; ii. a halogenide of niobium with nitrogen and methane togive NbNl_(1-x) C_(x) with x being less than or equal to 0.5; iii. ahalogenide of niobium with a halogenide of aluminum to give Nb₃ Al; iv.a halogenide of niobium with a halogenide of silicon to give Nb₃ Si; v.a halogenide of niobium with a halogenide of germanium to give Nb₃ Ge;vi. a halogenide of niobium with a halogenide of tin to give Nb₃ Sn;vii. a halogenide of vanadium and a halogenide of gallium to give V₃ Ga;or viii. a halogenide of vanadium with a halogenide of silicon to giveV₃ Si; b. incorporating the product particles produced in step a) in ametallic matrix with distances between said particles between 20 and 500A by suspending the superconnecting particles in an electrolyte andcoprecipitating a metal powder selected from the group consisting ofcopper, silver, gold or aluminum and the superconducting particles bygalvanic action onto an electrically conducting surface.
 15. The methodof claim 14, wherein said electrically conducting surface is a metallicsurface.